The present invention relates to a broadcast signal decoder and more particularly to a method and apparatus for decoding a facsimile and radio teletype broadcast signals.
Radio-facsimile broadcasts of weather charts and satellite photographs are transmitted in a range between 2-30 MHz by an international network of short wave radio stations. Many of these broadcasts are provided for transmission to ships for maritime navigational purposes. To decode radio facsimile transmissions, a standard short wave receiver is set to receive single side band SSB signals in the upper side band USB. Information is encoded on the transmissions by a frequency shift modulation FM of a center frequency of about 1900 hertz with a frequency value for black of 1500 hertz and a frequency value for white of 2300 hertz with gray areas encoded between these extremes. The chart is transmitted one horizontal line at a time at a rate of 2 lines per second. The left edge of the chart is defined by a series of synchronization lines transmitted before the start of each map. These lines consist of forty (40) lines of 1500 hertz tone which is interrupted for a short period with 2300 hertz tone at the beginning of each line. These interruptions are used by the receiving equipment to define the edge of the map and synchronization is maintained by timing one half second for each line until the beginning of the next chart. The transmitting stations also send an index selection signal of alternating 1500 hertz and 2300 hertz for five seconds to define the start of the transmission and a five second transmission of alternating 1500 hertz and 2300 hertz signals followed by ten seconds of a continuous 1500 hertz signal to define the end of the transmission that may be used automatically turn the recording equipment on and off.
Known radio facsimile receivers include a standard short wave receiver and an analog facsimile recorder that typically requires chemically treated or coated paper for recording the facsimile weather charts and diagrams. Such analog facsimile recorders typically weigh ten pounds or more and are designed to operate from 115 volts AC with the power consumption of 30 watts or more.